1. Field of the Invention
The present invention relates generally to holographic devices and methods for the fabrication thereof, and more particularly to holographic devices including single or multiple holographic elements, where the holographic elements are fabricated utilizing electromagnetic energy having a wavelength at which the media comprising the holographic elements is suitable for impressing a holographic grating pattern, wherein the holographic grating pattern that is thereby created is utilized to diffract a beam of electromagnetic energy having a wavelength at which the media is not necessarily suitable for recording, in a desired manner.
2. Brief Description of the Prior Art
Laser diodes offer significant advantages over other laser sources in efficiency, size and cost, but suffer inferior optical characteristics. Their beams diverge, have an asymmetric cross-section, and are often astigmatic. These deficiencies must be corrected to comply with many of the current applications of laser diodes in communication, data storage and imaging. Since refractive optics corrective systems are bulky and expensive, other approaches have been utilized, including: integrated optics lenses or gratings, waveguide optics and holographic elements. Computer generated gratings have good optical performance but suffer relatively low efficiencies and require sophisticated manufacturing equipment. Although the holographic approach offers small dimensions and a range of beam correcting capabilities, it is difficult to implement. High quality photographic recording media are not available for laser diode wavelengths, and recording with shorter wavelengths introduces severe aberrations. Schemes for alleviating such recording-to-readout wavelength shift aberrations, have been limited to spherical light sources, without any correction for deficiencies in the input beam, as described in G. Hatakoshi and K. Goto, Appl. Opt. 24, 4307 (1985); H. Chen, R. R. Hershey, and E. Leith, Appl. Opt. 26, 1983 (1987); and Y. Amitai, A. A. Friesem, and V. Weiss, J. Opt. Soc. Am. A 7, 80 (1990). Another method is limited in practice to large f-numbers, as described in H. P. Herzig, Opt. Comm. 58, 144 (1986). Furthermore, as diffractive devices, these elements suffer strong chromatic effects. This is a major drawback in laser-diode applications since the output frequency of these devices can vary significantly with operating conditions as well as aging. To compensate for the chromatic properties, multiple element systems, employing two or three diffractive elements, or a hologram-refractive element combination have been proposed, as taught in F. Yamagishi, M. Kato, S. Maeda, and T. Inagaki. Proc. SPIE 1334, 182 (1990); and Y. Ono, Y. Kimura, and S. Sugama, NEC Res. Develop. 89, 39 (1988). A method for recording high-efficiency, low-aberration collimators for spherical light sources with large recording-to-readout wavelength shifts was described by Y. Amitai and J. W. Goodman in Appl. Opt. 30, 2376 (1991).